tunnel objects are tunnels, encapsulating packets in IP packets and then
sending them over the IP infrastructure. The encapsulating (or outer) address
family is specified by the -f option. The default is IPv4.

set the fixed local address for tunneled packets. It must be an address on
another interface of this host.

ttl N

hoplimit N

set a fixed TTL (IPv4) or hoplimit (IPv6) N on tunneled packets.
N is a number in the range 1--255. 0 is a special value meaning
that packets inherit the TTL value. The default value for IPv4 tunnels is:
inherit. The default value for IPv6 tunnels is: 64.

tos T

dsfield T

tclass T

set the type of service (IPv4) or traffic class (IPv6) field on tunneled
packets, which can be specified as either a two-digit hex value (e.g. c0)
or a predefined string (e.g. internet). The value inherit causes
the field to be copied from the original IP header. The values
inherit/STRING or inherit/00..ff
will set the field to STRING or 00..ff when tunneling
non-IP packets. The default value is 00.

dev NAME

bind the tunnel to the device NAME so that tunneled packets will
only be routed via this device and will not be able to escape to another
device when the route to endpoint changes.

nopmtudisc

disable Path MTU Discovery on this tunnel. It is enabled by default. Note
that a fixed ttl is incompatible with this option: tunneling with a fixed
ttl always makes pmtu discovery.

ignore-df

enable IPv4 DF suppression on this tunnel. Normally datagrams that exceed
the MTU will be fragmented; the presence of the DF flag inhibits this,
resulting instead in an ICMP Unreachable (Fragmentation Required) message.
Enabling this attribute causes the DF flag to be ignored.

key K

ikey K

okey K

( only GRE tunnels ) use keyed GRE with key K. K is
either a number or an IP address-like dotted quad. The key
parameter sets the key to use in both directions. The ikey and
okey parameters set different keys for input and output.

csum, icsum, ocsum

( only GRE tunnels ) generate/require checksums for tunneled
packets. The ocsum flag calculates checksums for outgoing packets.
The icsum flag requires that all input packets have the correct
checksum. The csum flag is equivalent to the combination icsum
ocsum.

seq, iseq, oseq

( only GRE tunnels ) serialize packets. The oseq flag
enables sequencing of outgoing packets. The iseq flag requires that
all input packets are serialized. The seq flag is equivalent to the
combination iseq oseq. It doesn't work. Don't use it.